With the increased demand for mobile wireless services, network operators are exploring cost efficient ways to introduce new network services along with existing cellular services on the same carrier frequency. One such service is Multimedia Broadcast Multicast Service (MBMS). MBMS provides point to multipoint communication where packets are transmitted from a single source to multiple destinations such as MBMS user equipments (UEs) belonging to a multicast group.
In one implementation, MBMS is deployed within a Long Term Evolution (LTE) based network through the LTE Orthogonal Frequency Division Multiplexing (OFDM) air interface. As specified in LTE communication standards, a set of subframes of a carrier frequency can be assigned for the MBMS services. In the MBMS transmission mode, the broadcast/multicast data is synchronously transmitted by multiple cell sites within a geographical area on the same set of subframes of a carrier frequency and the geographical area is termed as a Multicast Broadcast Single Frequency Network (MBSFN). As used herein, a “cell” is a land area served by at least one radio unit, known as a “cell site”. Furthermore, a “network node”, a “base station” (BS) and “evolved Node-Bs” (eNB) are understood to be the same. Even further, a network node, BS or eNB are understood as possibly supporting multiple cell sites to provide coverage within multiple cells, by providing a common interface to all the supported cell sites' equipment to the evolved packet core network. A “network entity” refers herein to one or more physical entities connected to a cell site via the common interface provided by the cell-site's eNB. A network entity can be separate from an eNB, an eNB or distributed among several physical entities. A “serving cell site” is a cell site with which a UE has established a radio resource control (RRC) connection and that is actively listening to the DL system broadcast. Further, multiple cell sites may be supported by an evolved Node-B (eNB) or Base Station (BS). In particular, an eNB or BS act as the General Packet Radio Service (GPRS) tunnel point (GTP) end point for data and control paths associated with all the UEs actively communicating with any of the cell sites that belong to the eNB/BS from the evolved packet core (EPC).
Subframes for MBMS services, i.e., MBSFN subframes, are semistatically configured by the network based on the type of broadcast/multicast service and the associated Quality of Service (QoS) requirements. The MBMS UEs which are interested (and subscribed) to broadcast/multicast services listen to assigned MBSFN subframes. MBSFN reference signals are also transmitted as part of the assigned MBSFN subframes by all the cell sites within the MBSFN area in which the reference signals are generated based on the MBSFN area identity (NIDMBSFN). The data symbols containing the broadcast or multicast content are transmitted using a modulation and coding scheme (MCS) value specified in a System Information Block 13 (SIB13) message.
The MCS and number of MBSFN subframes used for transmitting the MBMS content within an MBSFN area are determined in order to satisfy the QoS requirement for the specific service(s) for all the UEs actively listening to the service(s). When an MCS with higher spectral efficiency, i.e., more number of bits per modulation symbol, can reliably deliver transmitted content to all the UEs actively listening within the MBSFN area, fewer subframes need to be assigned for MBMS service. The rest of the subframes of the carrier frequency can be used for unicast cellular traffic, thus increasing the overall spectral efficiency. The MCS for a MBMS service is dependent on the lowest signal to interference plus noise ratio (SINR) experience by the UEs within the MBSFN area. A network entity which has information about the minimum Signal-to-Interference Noise Ratio (SINR) within a MBSFN area can decide the MCS for the MBMS transmission.
MBMS UEs receive content from cell sites that are using the same transmission parameters. The number of cell sites transmitting broadcast or multicast content is transparent to MBMS UEs.
If a UE receives the same signal from N cell sites, the received resource element (RE) at the UE-m, Rkl, can be expressed as follows:Rkl=Σi=0N-1√{square root over (ρi→mM)}Hkl(i→m)Skl+Nkl  (Eq. 1)where ρi→mM is the average received power at the UE-m from cell site-i for MBMS transmission, Hkl(i→m) is the channel weight on kth subcarrier of lth OFDM symbol, Hkl(i→m) can be represented as a zero-mean complex Gaussian variable with the following property:
                    ∑                  k          =          0                                                    N              RB              DL                        ⁢                          N              sc              RB                                -          1                    ⁢                          ⁢                                                            H              kl                        ⁡                          (                              i                →                m                            )                                                2              =    1    ,NRBDL and NscRB are the number of resource blocks (RBs) in the downlink (DL) and the number of subcarriers per RB respectively, Rkl is the received RE at the kth subcarrier of the lth OFDM symbol. Skl is the transmitted symbol on the kth subcarrier of the lth OFDM symbol and Nkl represents the thermal noise observed on the kth subcarrier of the lth OFDM symbol and any unwanted interference.
As mentioned above, the multi-cell synchronous transmission is transparent to the MBMS-UE, in part, because the reference symbols transmitted by the cell sites are the same, such that the UEs cannot distinguish the transmission from different cell sites. Therefore, the above equation can be re-written as follows:Rkl={tilde over (H)}kl(i→m)Skl+Nkl  (Eq. 2){tilde over (H)}kl(i→m)=Σi=0N-1√{square root over (ρi→mM)}Hkl(i→m)  (Eq. 3)The signal received at the MBMS-UE-m experiences the composite channel effects represented by {tilde over (H)}kl(i→m). As illustrated in FIG. 1, the received power from each cell site can be represented as follows:
                              ρ                      i            →            m                    M                =                              P            i            M                                L                          i              →              m                                                          (                  Eq          .                                          ⁢          4                )            where PiM is the average transmit power level at cell site-i over the MBSFN transmissions and Li→m represents the path-loss from cell site-i to UE-m.
The relationship of ρ to the reference signal received (RSRP) measurements by a non-MBMS UE is described next. As used herein, a non-MBMS UE is a UE which is not actively listening to MBMS services. Non-MBMS UEs that are connected to cell sites for unicast services measure the downlink (DL) signal quality over the transmitted cell site specific reference signals (which are transmitted as part of the normal subframes and these references signals are different from the MBSFN reference signals), and report the measurement back to their respective serving cell site. The channel quality may, for example, be measured as a Reference Signal Receive Power (RSRP) or a Reference Signal Receive Quality (RSRQ). The RSRP can be measured over the reference signal transmitted on antenna port-0 (and, optionally, also over antenna port 1) by the serving cell site and the one or more neighboring cell sites. The reporting mechanism of the non-MBMS UEs may be triggered by the serving cell site. The RSRP is measured over OFDM symbols with Reference Symbol Resource Element (RSRE) and is the linear average over the power contribution of the resource elements that consist of cell site specific reference signals (CRS) within the considered measurement frequency bandwidth (UE implementation). RSRP can be determined over CRS port-0. If the UE can reliably detect that CRS port-1 is available, it may use CRS on port-1 in addition to CRS on port-0 to determine the RSRP. RSRP measurements that are measured at a non-MBMS UE-u, can be expressed mathematically as follows:
                                          RSRP            ⁡                          (                              i                →                u                            )                                =                                                    P                i                            ⁢                              E                [                                                      ∑                                          k                      =                      0                                                                                                                N                          RB                          DL                                                ⁢                                                  N                          sc                          RB                                                                    -                      1                                                        ⁢                                                                          ⁢                                                                                                                                    H                          ki                                                ⁡                                                  (                                                      i                            →                            u                                                    )                                                                                                            2                                                  ]                                      =                                          ρ                                  i                  →                  u                                            ⁢                            ⁢                                                          ⁢              or                                      ⁢                                  ⁢                                  ⁢                              i            =            0                    ,          1          ,          …          ⁢                                          ,                      N            -            1                                              (                  Eq          .                                          ⁢          5                )            where E[ ] is the expectation operator. In the above equation, E[ ] indicates the averaging over the OFDM symbols, i.e., l. ρi→u represents the RSRP measured with respect to cell site-i of non-MBMS UE-u. In one embodiment, the RSRP measurements are included in the channel quality measurement reports received at the serving cell site.
Since there is no RSRP or other channel quality measurement feedback from MBMS UEs, existing MBMS solutions rely on extensive drive tests to guarantee reception throughout the MBSFN area. These drive test involve physically driving across the coverage area where the MBMS services will be provided, and measuring the signal quality at various locations using specialized test equipment. The measurements are stored and later evaluated to determine which MCS is best for meeting a promised Quality of Service (QoS) for a MBMS service within the coverage area. While these extensive drive tests may provide a method for configuring MBSFN areas and subsequent assignment of the MCS, these drive test based configurations are very time consuming and very expensive due to the fact that technicians have to physically drive around the coverage area to take measurements. Further, each reconfiguration of the MBSFN area and assigned MCS may disadvantageously require respective drive tests.